1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotography system, particularly relates to an image forming apparatus of a copier, a printer, a facsimile or the like.
2. Description of the Related Art
As an image forming apparatus adopting an electrostatic recording system or an electrophotography recording system, a copier, a printer, a facsimile apparatus or the like has been known. As an image forming apparatus of this kind, there is widely used a so-called roller type fixing apparatus for fixing a toner image onto a sheet by a pair of rollers brought into press contact with each other. According to the roller type fixing type apparatus, by bringing a fixing roller and a pressing roller of silicone rubber or the like into press contact with each other, a nip in a planar shape is formed at a contact face of the two rollers and fixing is executed by applying pressure and heat to the sheet at the nip portion.
At the nip portion, a rubber structure of the two rollers is always subjected to compressive strain. Therefore, when a stationary state of the fixing roller is continued for a long period of time, bonding of the rubber structure may be destructed by exceeding a plastic limit and the strain may not be recovered to nullify. Such a state is referred to as a compression set (compressive permanent set). The rubber roller causing the compression set is bent and therefore, the rubber roller cannot form an image or carry the sheet correctly in the fixing apparatus, and jamming, color shift, sheet skewing or the like is brought about. Hence, there has been proposed a method of preventing the compression set by shifting the nip position by rotating the rollers at constant time intervals in a standby mode in which the fixing roller is stationary (Japanese Patent Unexamined publication No. HEI 4-74708).
Meanwhile, in recent years, as is seen also in international standards of energy star and the like, reduction and the efficient formation of power consumption of a power consuming apparatus has strongly been promoted. Also an image forming apparatus is not exceptional and there has been devised an apparatus of adopting a measure of reducing power consumption of shifting from a standby mode to an energy saving mode of reducing power consumption when the image forming apparatus is not operated for a constant period of time in a standby mode in order to reduce power consumption at standby.
For example, when the above-described nip position change control executed at constant time intervals is adopted in such an energy saving mode, since an initial temperature of the fixing roller in shifting to the energy saving mode is not taken into consideration, there is a case in which a frequency of executing nip position change operation becomes excessively large or a case in which the frequency becomes excessively small. That is, there is a possibility that the nip position change operation is not carried out efficiently and properly.